Proportioning valve assemblies have been utilized for many years in conjunction with vehicle braking systems. Depending on the fluid pressure generated by the master cylinder of the brake system, the proportioning valve may operate to restrict fluid pressure being communicated through the brake lines to the rear wheel brakes, and thereby prevent the communication of excessive brake pressure which would cause lockup of the rear wheels and skidding of the vehicle. Typical prior proportioning valve assemblies are those such as illustrated in U.S. Pat. No. 4,213,655 wherein the body of the proportioning valve includes at the inlet end an opening in which is machined a groove for positioning one end of a spring, the spring biasing a valve member or poppet downwardly toward the differential piston of the proportioning valve assembly. The machining of the groove in the very small inside diameter of the inlet opening is difficult and expensive. More importantly, the machining of the opening and assembly of the spring and poppet encompass an accumulation or "stack-up" of tolerances which affect the kneepoint of the output pressure curve of the proportioning valve assembly. The knee of the output curve is determined by the spring load at the position of contact and seal of the differential piston with the poppet. It is desirable to eliminate the stack-up of tolerances so that the proportioning valve assembly operates in accordance with its intended design characteristics, and also to eliminate the more expensive manufacturing costs inherent therein.